MarketPress shouldn't be doing this on it's own, my guess would be that there's a similar function or script in one of your other plugins or theme that uses same class name or ID of the MP popup element and it's firing it up on your site.

Can you perform a basic troubleshooting by disabling your other plugins and switching to your default theme?
If it doesn't show popup after that enable your plugins one by one to see which one is causing it and let us know so we can debug this further.

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